With the rising cost of wood and the shortage of mature trees, it is desirable to find substitutes for wood. A growing market has emerged for the use of wood polymer composites (WPCs) to replace traditional wood products, including pressure-treated lumber in applications such as decking, window casing, fencing, automobile interiors and pallets. WPCs typically consist of mixtures of thermoplastic materials with cellulose, such as wood particles in the form of sawdust. WPCs may be used in many of the same applications as an all-wood product but offer advantages of providing flame resistance, as well as enhanced resistance to rot, resistance to attack by insects, and resistance to deterioration due to the effects of moisture and sunlight. WPCs may have the same workability as wood, may be splinter-free, and may be capable of being colored in bulk as opposed to wood, which typically can only be surface stained or painted. The encapsulation of cellulosic fibers in a polymeric matrix is known, having previously been disclosed, for example, in U.S. Pat. Nos. 3,836,412; 4,228,116; 4,248,743; 4,376,144; and 4,792,020, each of which are incorporated by reference in their entirety.
In general, consumers prefer WPC over pressure-treated lumber and want products having extended lifetimes (for example, at least for 25 years) with minimal maintenance.
Current WPC products offer significant advantages over pressure-treated lumber for maintenance over the life of the product. However, WPC products are subject to the same environmental factors, including staining, scuffing, scratching, marring and fading, as pressure treated lumber, so it is desirable to limit the susceptibility of the WPC to those factors. Most of these issues relate to the surface of the WPC and not the bulk of the material.
It is desirable to provide additional wear resistance and ink stability to WPCs. There have been attempts to modify the surface of WPCs, such as the use of surface or cap layers over a bulk or core WPC layer. Previous surface layers include an acrylonitrile-styrene-acrylate surface or cap layer (see U.S. Patent Application Publications 2006/0147693 and 2008/0128933, each of which are incorporated by reference in their entirety) and use of a coating applied on the surface of the WPC (e.g. special paint). U.S. Patent Application Publication 2005/194585, which is incorporated by reference in its entirety, describes wood articles made from wood substrates (i.e. natural wood, pressure treated wood, or laminated wood) and ionomers cross head extruded onto the wood substrate. When a coating has been used, the surface of the WPC frequently requires pretreatment to allow the application of the coating layer.
One attempt at providing additional wear resistance includes Zuri Decking by Royal Building Products. Zuri decking has a printed woodgrain image between a PVC core and clear PMMA wear layer. The article formed by the Zuri process undesirably is prone to stain/whiten when exposed to plasticizers commonly found in PVC floor mats, garden hoses, etc.
Other systems providing coated systems include interior laminate flooring. These types of floors will have woodgrain images typically printed on paper or PVC films with coatings applied for wear and scratch resistant properties. These coatings applied to indoor flooring suffer from the drawback that they degrade in exterior applications and are not capable of providing fade resistance to meet industry acceptable lifetimes.
Wood polymer composite systems that provide enhanced wear resistance and ink stability are desirable in the art.